Reproduction in women depends upon the dynamic interaction of several compartments of the female reproductive system. The hypothalamic-pituitary-gonadal axis orchestrates a series of events affecting the ovaries and the uterine-endometrial compartment that leads to the production of mature ova, ovulation, and ultimately appropriate conditions necessary for fertilization. Specifically, luteinizing hormone-releasing hormone (LHRH), released from the hypothalamus, initiates the release of the gonadotropins, luteneizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary. These hormones act directly on the ovary to promote the development of selected follicles by inducing granulosa and theca cell proliferation and differentiation. FSH stimulates aromatization of androgens to estrogens and increases the expression of LH receptors in the theca cells. The follicles, in turn, secrete steroids (estradiol, progesterone) and peptides (inhibin, activin). Estradiol and inhibin levels progressively increase during the follicular phase of the menstrual cycle until ovulation. Inhibin decreases FSH secretion from the pituitary gland, while estradiol acts on the hypothalamus and pituitary to induce the LH surge in mid-cycle, which results in ovulation. Afterwards, the post-ovulation, ruptured follicle forms the corpus luteum, which produces progesterone. Ovarian hormones, in turn, regulate the secretion of gonadotropins through a classical long-loop negative feedback mechanism. The elucidation of these control mechanisms has provided opportunities for the development of effective strategies to control fertility, including both enhancement of fertility and contraception. For recent reviews of FSH action see: “FSH Action and Intraovarian Regulation”, B. C. J. M. Fauser Editor, Parthenon Publishing Group, Vol. 6, 1997 and A. J. Hsueh, T. Bicsak, X.-C. Ja, K. D. Dahl, B. C. J. M. Fauser, A. B. Galway, N. Czwkala, S. Pavlou, H. Pakoff, J. Keene, I. Boime, Granulosa “Cells as Hormone Targets: The Role of Biologically Active Follicle-Stimulating Hormone in Reproduction”, Rec. Prog. Horm. Res., 45, 209-227, 1989.
Current hormonal contraception methods are steroidal in nature (progestins and estrogens) and modulate long-loop feedback inhibition of gonadotropin secretion, as well as affecting peripheral mechanisms such as sperm migration and fertilization. The development of specific antagonists of the receptor for FSH (FSH-R) would provide an alternative strategy for hormonal contraception. Such antagonists would block FSH-mediated follicular development leading to a blockade of ovulation, thereby producing the desired contraceptive effect. Support for the effectiveness of this strategy is provided by the mechanism that causes resistant ovary syndrome which results in infertility in women. The infertility experienced by these women is the result of non-functional FSH receptors (K. Aittomaki, J. L. D. Lucena, P. Pakarinen, P. Sistonen, J. Tapainainnen, J. Gromoll, R. Kashikari, E.-M. Sankila, H. Lehvaslaiho, A. R. Engel, E. Nieschlag, I. Huhtaniemi, A. de la Chapelle “Mutations in the Follicle-Stimulating Hormone Receptor Gene Causes Hereditary Hypergonadotropic Ovarian Failure” Cell, 82, 959-968, 1995). This approach to contraception may be applicable to men as well, since idiopathic male infertility seems to be related to a reduction in FSH binding sites. In addition, men with selective FSH deficiency are oligo- or azoospermic with normal testosterone levels and present normal virilization (G. Lindstedt, E. Nystrom, C. Matthews, I. Ernest, P. O. Janson, K. Chattarjee, Clin. Lab. Med., 36, 664, 1998). Therefore, orally active, low molecular weight FSH antagonists may provide a versatile novel method of contraception. Such an antagonist could be expected to interfere with follicle development and thus ovulation, while maintaining sufficient estrogen production and beneficial effects on bone mass.
FSH actions are mediated by binding of the hormone to a specific transmembrane G protein-coupled receptor exclusively expressed in the ovary, leading to activation of the adenyl cyclase system and elevation of intracellular levels of the second messenger cAMP (A. Mukherjee, O. K. Park-Sarge, K. Mayo, Endocrinology, 137, 3234 (1996)).